The I-551 stamp in the passport is the same as having a green card. It shows that you are a lawful permanent resident and free to work. When the passport is stamped, the USCIS will indicate how long it is valid for. Usually it is stamped for a year to hold you over until the green card comes in the mail.

Once you get the stamp in your passport, they will ask for your EAD back. The stamp says clearly EMPLOYMENT AUTHORIZED. A while after receiving the stamp, you will get a plastic card (green card) in the mail.